Finding and neutralizing underground terror tunnels in the Gaza Strip has become a top priority as the result of Operation Protective Edge, and experts say ground radar could help. The Israel Defense Forces acknowledges that it is testing some systems without describing them – and the “wall of radar” might be one of them.

A senior IDF officer told journalists during the fighting that the army is looking to deploy two or three layers of technology in combination with underground barriers on the Israel-Gaza border. The goal is to create a system to detect and deter the “attack tunnels.”

During the war, the scope of the tunneling became clear when Hamas gunmen emerged from tunnels to kill 11 Israeli soldiers. Prime Minister Benjamin Netanyahu said civilians were targets as well, and the prospect of terrorists emerging from tunnels and attacking border communities, possibly kidnapping Israelis, frightened many.

Israeli defense planners have known about the extensive Hamas tunnel network for years, but the IDF did not find an effective way to detect and destroy them until the last round of fighting erupted, and Israel sent ground forces into Gaza to blow up the tunnels.

The goal now is to find the tunnels without sending troops into Gaza. Radar is one idea, but an Israeli expert says more work needs to be done on it.

Ground-penetrating radar, known as GPR, is among the most promising technological responses to the tunnels, Israeli and American experts say. The radar – which can “see” into the ground – has been used from the surface to search for smuggling tunnels under the US-Mexico border. Radar installations are also installed in deep holes in the ground to search for attack tunnels under the Korean Demilitarized Zone.

The experts say the Korean type of of cross-borehole ground-penetrating radar could be installed along the border to create a permanent detection barrier – deep enough to spot any tunnel Palestinians militants could dig. The barrier could be monitored for changes from a remote center, and in combination with other technologies could provide the best method of securing the border.

“I am thinking about a kind of detection barrier that will detect tunnels down to the water table [the level below which the ground is saturated with water],” said Prof. Amos Frumkin, a geologist and head of the Cave Research Unit at the Hebrew University of Jerusalem. He uses ground-penetrating radar in his work. “We are not there exactly technologically, but I am quite sure it’s possible. It needs some trial and error and some study, but this would be the solution, I believe.”

American attempts

Ground-penetrating radar works by sending radio waves underground and receiving them when they are reflected back. Different materials reflect the waves differently. A computer notes the travel time and strength of the reflected waves and generates a 3-D image based on the information.

A version of the radar that is pushed along the ground in a machine that looks like a lawnmower is commonly used for underground surveying in engineering and scientific research. It has also been used to detect buried explosives in war zones like Iraq and Afghanistan and to locate evidence in police investigations.

‘It should not be too difficult to detect these tunnels using proper ground-penetrating radar’

US border police use ground-penetrating radar from the surface to look for smuggling tunnels from Mexico. The tunnels are mostly for moving people and drugs into the United States, but authorities worry they could also serve to bring in weapons or to launch terrorist attacks.

Some 170 tunnels have been discovered along the US-Mexico border – all as the result of old-fashioned police work or by chance, not ground-penetrating radar or any other detection technology. In hopes of changing that, the US Department of Homeland Security reported in 2009 that it was working on adapting the radar so that a truck could drag it along the border in a trailer. The department declined to provide updates on its progress.

A limitation of ground-penetrating radar is that even in ideal conditions, it only provides an accurate image from the surface up to a depth of about 15 meters. The known tunnels in Mexico are as much as 27 meters below ground.

In the DMZ between North and South Korea, four tunnels have been found from the north running as deep as 160 meters below ground. The South Korean army – previously with guidance from the US Army Corps of Engineers – has on an ad hoc basis used cross-borehole ground-penetrating radar to look for tunnels as deep as 600 meters, according to an expert involved in the project who asked not to be identified because he was not authorized to speak about it to reporters. The US Army and Defense Department declined to comment, citing security concerns.

In cross-borehole ground-penetrating radar, pairs of narrow holes are drilled deep into the ground and antennae are lowered into them — one for sending and the other for receiving the signals. From the boreholes, the radar can provide an image all the way down to the water table. However, there have been no reports of tunnels being found this way in the DMZ.

“I have seen first-hand that you can use radar in drill holes to get better definition and resolution of objects in the subsurface,” said Prof. Jeffrey Daniels, an applied geophysicist at The Ohio State University, who specializes in underground surveying. “You just have to drill down to the target.”

Still, he says, in his experience the radar works only about half the time and sometimes gives false positives. It requires expert interpretation and can be weakened by wet or clay soil and distorted by objects and other signals.

Deeper solutions

The experts say that if the radar can be used and improved anywhere, it is in conditions like those on the Israel-Gaza border. The ground in that region is dry and sandy, and thanks to the proximity of the Mediterranean Sea, the water table is relatively high– about 100 meters below the surface.

Because Gaza’s border with Israel is just 51 kilometers long, it is feasible to station antennas in boreholes all the way along it, the experts say. Information could be transmitted from all the antennas to a remote computer center to be processed and monitored. A baseline image of the “normal” state of the ground underneath the border could be used to test and improve the accuracy of the system.

“Based on the soil properties in Gaza strip, as well as size and depth of these tunnels, it should not be too difficult to detect these tunnels using proper ground-penetrating radar,” said Prof. Chi-Chih Chen, an electrical and computer engineer at The Ohio State University, who has done research on detecting tunnels with the radar. “I would be very surprised if this has not been tried by Israel yet.”

The senior IDF officer told journalists that if the systems being tested on the border prove successful, the IDF would have to invest 1.5 billion to 2.5 billion shekels ($420 million to $700 million) to deploy them. The IDF has conducted thousands of trials in some 700 pilot programs in the past few years to find the right combination of technologies, the officer said. Ground-penetrating radar is doubtless among them, according to the experts, though they had no insider knowledge.

Given its limitations, they say, ground-penetrating radar should be augmented on the border with other technologies, including electrical methods, seismic, gravimetric, and acoustic systems. Israel’s previous testing has reportedly focused on seismic systems, which shake the earth and read the reverberations on the rebound.

No matter what combination of technologies is deployed, the experts warn, they will not work all the time or forever. One threat is that Israel’s enemies will eventually develop techniques for digging below the water table, complicating detection efforts.

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